1. Field of the Invention
The invention relates to an oscilloscope probe comprising a transistor amplifier on a semiconductor substrate using integrated-circuit technology with an input-voltage divider.
2. Related Technology
Active probes for earthed or differential oscilloscopes contain an amplifier acting as an impedance converter, which taps the signal to be measured in a high-ohmic manner via the probe tip and, at the output, supplies the signal via a corresponding high-frequency cable to the input of the oscilloscope with a characteristic impedance generally of 50 ohms. An input-voltage divider, which is constructed from resistors and capacitors and is used to increase the linear range of the amplifier and to compensate its input capacity, is generally connected upstream of this amplifier. This input divider represents the limiting factor for the further miniaturization of this type of active probe. Moreover, it generates parasitic effects, which limit the bandwidth of active probes of this kind. Finally, an amplifier of this type can cause oscillations, if it is not matched at the input in a low-ohmic manner. This occurs, if the amplifier input is connected to the input divider via a bonding wire, which provides a high impedance for high-frequencies.
All of the known active probes for oscilloscopes use voltage dividers, which are constructed either of discrete components or realized using thick-layer or thin-layer technology (for example, according to U.S. patent specifications U.S. Pat. No. 5,172,051, U.S. Pat. No. 6,483,284, U.S. Pat. No. 6,949,919, U.S. Pat. No. 6,982,550, U.S. Pat. No. 5,061,892, U.S. Pat. No. 5,796,308, U.S. Pat. No. 6,720,828, U.S. Pat. No. 6,967,473 or U.S. Pat. No. 6,828,769). The elements of the voltage divider in this context are disposed on a ceramic or printed-circuit-board substrate together with the amplifier chip, which is provided as an integral component. The adjustment of the voltage divider is implemented via potentiometers, varactor diodes or by laser adjustment of resistor or capacitor surfaces on the substrate. With all of these known arrangements, a large part of the substrate surface is occupied by the adjustable voltage divider. Furthermore, parasitic elements of the voltage divider generate undesirable frequency responses, which reduce the bandwidth to a few GHz. Such parasitic elements are primarily inductances and capacitances of the relatively-large, thick-layer elements in the millimeter range and the inductance of the bonding wires between the elements. In the case of a structure built up from discrete components, the influence of parasitic elements is usually even more dominant.